Easy. Djokovic has it harder because he is not as good as Federer was relative to the rest of the field.

Without being disrespectful, Novak is merely a great tennis player at this stage. Federer, on the other hand, was a genius until the tour and age caught up with him. Novak plays essentially the same style as Murray, Nadal, Ferrer and almost everybody else, just a little bit better. If you watched tennis in 2003-2006 you knew immediately that Federer had (and still has) a style all his own.